Shape memory effect materials such as Nitinol (NiTi), or copper-zinc-aluminum brasses have been proposed for use in transducers such as actuators and relays. Simple electrothermal relays are known wherein a wire of Nitinol pulls a set of electrical contacts into engagement. Such devices have not been commercialized because of severe problems of element creep, power consumption, cycling rate due to cooling time and/or reliability because of tendencies to burn out.
A simple transducer known to the prior art is one wherein a length of shape memory wire, such as Nitinol, is disposed in series with a spring between a support means and a member to be actuated with a circuit for supplying electrical current through the Nitinol wire whereby the resistance of the wire causes the Nitinol wire to heat above its austenite finish temperature (i.e., transition temperature) so that the wire shortens in length and returns to its memory shape causing the movable end of the wire to move the armature or primary member to a selected position. Heat is removed from the wire by the termination of electrical current therethrough and cooling to ambient temperature at a rate depending upon the temperature difference between the heated wire and ambient. Other factors determining the rate of cooling of the wire include specific heat of the material of which the wire is made, mass and surface area, fluid convection, latent heat of transition, thermal conductivity and diffusivity.
An important limiting aspect of such a simple actuator is that when the electrical current through the shape memory element or wire is interrupted and then the wire cools by conduction, convection and/or radiation to the surrounding environment and the martensitic start temperature is reached, the shape memory element or wire becomes weaker and superplastic. The return spring then overcomes the internal resisting stress in the shape memory element or wire and returns it to the initial position. In other words, the removal of the actuating current which provides heat to the actuating wire simply allows the element to cool and the return motion or lengthening of the wire is a result of the spring in series with the wire.
A drawback of such a combination of elements is that the movable end of the transducer exerts a known force upon the primary or armature member being moved only when the shape memory element is energized or heated above its transition temperature. As the shape memory element cools, the movable end returns to its initial position rather slowly. In other words, the spring in series with the shape memory element applies a continuous force or stress to the element. Consequently, if the return spring strains the shape memory element before it is fully cooled, parts of the element may be plastically deformed and cold worked leading to eventual failure.